I. Field of the Invention
This invention relates to a surge absorber suitable for protecting electronic devices used for communication equipment, such as, facsimiles, telephone switchboards, modems, and the like, from surge voltages and continuous overvoltages or overcurrents. More particularly, it relates to a surge absorber which includes a surge absorbing element used for protecting the electronic devices from surge voltages and wire means for preventing abnormal and deleterious heating of the surge absorbing element when continuous overvoltages or overcurrents flow to the surge absorber.
II. Description of the Related Art
In the prior art, a conventional surge absorbing element, e.g., a gas charge tube, is parallel connected to an electronic device to be protected via a pair of input lines of the electronic device, and is designed to operate at a higher voltage than the operating voltage of the electronic device. Such a prior art surge absorbing element is a resistor having a high resistance when the voltage applied thereto is lower than the discharge voltage thereof and a resistance tens of ohms lower when the voltage applied thereto is higher than the discharge starting voltage thereof. Accordingly, when surge voltages, such as, lightning surges, etc., are instantaneously applied to an electronic circuit including the surge absorbing element and the electronic device, the surge absorbing element discharges to suppress the surge voltages, and serves to protect the electronic device from the surge voltages. However, when an overvoltage or overcurrent due to an accident, etc., is continuously applied to the electronic circuit, a certain amount of current continuously flows through the surge absorbing element. This results in the surge absorbing element being heated to high temperatures. The heat radiating from the surge absorbing element can cause the protected electronic device, as well as other electronic devices surrounding the surge absorbing element to catch fire.
A typical example would be an accident wherein the input lines of the electronic device contact the power lines thereof. While it does not usually happen that such overvoltages or overcurrents resulting from such accidents are continuously applied to the surge absorbing element, to achieve maximum safety, it has recently become desirable to take additional safety measures to avoid such accidental problems and fires caused thereby. An example of such maximum safety measures are those prescribed by Underwriter's Laboratories, Inc. of the U.S.A (UL) which requires a safety standard for surge absorbing elements so that they do not cause fire or electrical shock in communication equipment surrounding the surge absorbing element when continuous overvoltages or overcurrents are applied.
Japanese laid open patent application No. S63-18923 discloses a surge absorber which passes these safety standards. The disclosed surge absorber can prevent the abnormal heating of a surge absorbing element due to continuous overvoltages or overcurrents and thereby prevent electronic devices located close to the surge absorber from catching fire. This prior art surge absorber disclosed in this patent application is depicted in FIG. 4 and is composed of a surge absorbing element and a metal wire of a low melting point metal adhered to a surface of the surge absorbing element. The wire is electrically series connected to the surge absorbing element. As shown, surge absorber 40 has two lead pins 42 and 43 which pierce through an insulating base plate 41 of flameproof resin, e.g., polybutylene terephthalate. One end of lead pin 42 is welded to one end of a low melting point 0.25 mm diameter metal wire 49. One end of lead pin 43 is welded to a lead wire 17 of a surge absorbing element 14. The other end of the low melting point metal wire 49 is soldered to a lead wire 16 of surge absorbing element 14 by solder 28. Cylindrical glass 50 having an outer diameter of 5 mm, an inner diameter of 3.5 mm and a length of 10 mm is attached to base plate 41 to encase the surge absorbing element 14 and the low melting point metal wire 49. A casing or housing 45 of the same material as base plate 41 is further attached to the base plate 41. The respective other ends of the two lead pins 42 and 43 are inserted through holes 47 and 48 of a print circuit board 46 and then are soldered to the print circuit board 46.
A silver-lead (silver 7 wt.%) wire is used as low melting point metal wire 49. When a continuous overvoltage or overcurrent flows to this surge absorber, the low melting point metal wire 49 melts (blows) from the abnormal heating of surge absorbing element 14. This cuts the continuous current from flowing to the surge absorbing element and then the abnormal heating stops.
However, a problem with the above surge absorber is that the surge absorber must be changed for a new one due to the melting of the solder connecting the lead pins to the print circuit board after the low melting point solder blows and changing the surge absorber is troublesome. Another problem with this surge absorber is that it requires a sufficient amount of space to locate the low melting point metal wire close enough to the surge absorbing element so as to assure that it rapidly blows, and also needs a relatively wide space between the surge absorbing element and the inner wall of the casing to prevent a thermal modification of the casing due to the abnormal heating of the surge absorbing element. As a result, it is difficult to miniaturize the surge absorber.